Aluminum alloy in the form of sheet is a favored material for making lithographic plate ("lithoplate") and foil for electrical capacitors because of its cost effectiveness. However, lithoplate and capacitor foil must be properly grained or toughened, which involves "extending" the surface of the sheet or foil, as explained hereinafter. By "lithoplate" reference is made to the aluminum support material before it is coated with a photosensitive "resist". By "cost effectiveness" we refer to the number of prints of acceptable quality which can be made with a single resist-coated lithoplate before it must be replaced. By "graining" we refer to the roughening of a surface of a metal surface for a number of purposes, such as cleaning, preparing a surface for bonding to another surface, annealing and other property changing effects. And, as disclosed hereinafter, such processes can be performed on an in-line, continuous basis.
Graining an aluminum sheet is the first step towards providing photoresist-coated sheet with the requisite hydrophobic and hydrophilic characteristics which generate image and non-image areas. Though an aluminum alloy is used, commercial lithoplate of aluminum alloy is referred to as "aluminum" sheet or foil, for brevity, partially because nearly pure aluminum, such as 1050 alloy (99.5% pure) is a preferred material for electrochemically etched lithoplate, and partially because pure aluminum is known to be an impractical material for lithoplate.
Lithoplate for off-set printing is typically provided on one side with a layer of an organic composition which is light-sensitive. This layer permits the copying or reproduction of a printing image by a photomechanical process. Upon formation of the printing image, the grained supporting material on which the layer is deposited carries the printing image-areas and, simultaneously forms, in the areas which are free from an image, the hydrophilic image-background for the lithographic printing operation.
The grained supporting surface, laid bare in the non-image area, must be so hydrophilic that it exerts a powerful repulsion of greasy printing ink. The photosensitive layer must adhere strongly to the grained aluminum support, both before and after exposure. It is therefore essential that the grained support be highly stable, both mechanically, from an abrasion standpoint, as well as chemically, particularly relative to alkaline media.
To provide the hydrophobic and hydrophilic characteristics, a grained aluminum sheet is uniformly coated with a photosensitive "resist" composition which is exposed to actinic radiation beamed onto the resist through an overlay which corresponds to the image to be printed. Areas which are comparatively more soluble following irradiation must be capable of being easily removed from the support, by a developing operation, to generate the hydrophilic non-image areas without leaving a residue. The support which has been laid bare must be strongly hydrophilic during the lithographic printing operation, and be able to exert an adequately repelling effect with respect to greasy printing ink.
The cost of producing lithoplate includes the cost of producing foil of an affordable alloy, the foil preferably having a highly uniform microstructure, such as that obtained with controlled fabricating practices, e.g., rolling and thermal treatment to assure uniform response to electrochemical etching. The conventional wisdom has been: the more uniform the microstructure of controllably grained foil, the more uniformly the lithoplate will grain and thus be better suited for use as lithoplate. Lithoplate requires a near perfect surface for printing purposes whereas sheet used for resistance welding does not require such a perfect surface.
In addition to 1050 alloy, other widely used alloys are 3003, 1100 and 5XXX, the latter being specifically produced for the production of lithoplate, as disclosed in U.S. Pat. No. 4,902,353 to Rooy et al, the disclosure of which is incorporated by reference thereto as if fully set forth herein. Though the cost of such alloys is not high relative to the value of the printed material generated, lithoplate is nevertheless deemed costly, and the on-going challenge is to produce more cost-effective lithoplate.
The cost of lithoplate in large part lies in the cost of graining aluminum sheet so that it is free from imperfections, and will provide adequate resolution of the print to be made, as well as many hundreds, if not thousands of prints, before one must change the lithoplate in a printing press. Such imperfection-free graining, at present, is preferably accomplished by choice of an alloy having a microstructure which is particularly well-adapted to electrochemical graining which is closely controlled by a bath composition and the narrowly defined process conditions of its use. Together these result in highly uniform graining or roughening. Not only is the optimum aluminum alloy expensive because of the special processing which may be required to obtain the desired microstructure, in reference to the topography of the printing surface, but there is also the necessary close control for electrochemical graining, and formulating and maintaining a chemical bath. Disposing of exhausted bath compositions further adds to the expense.
Such considerations militate towards finding a non-chemical solution to the problem of graining a metal sheet or foil and to other preparations such as cleaning and annealing. But non-chemical graining, that is, mechanical graining, is generally accepted as being too non-uniform, not only because it is relatively coarse compared to electrochemical etching, but also because it is difficult to control. The on-going search is for a solution to the problem of providing controllably grained surfaces without using an electrochemical process.
One such controllable graining process is disclosed in the above U.S. Pat. No. 5,187,046 issued to the present assignee. The disclosure of that patent is directed to the use of a single or multiple individual electrodes that in at least one embodiment apply helical traces or a raster type graining to a sheet of material clamped on a rotating drum. Such a process is relatively slow and the graining effect is somewhat non-uniform.